What characterizes a 1st degree heart block?

A first-degree heart block is characterized by a prolonged PR interval on an ECG. The PR interval represents the time it takes for electrical impulses to travel from the atria through the AV node to the ventricles. In first-degree heart block, this conduction time is longer than normal, typically exceeding 0.20 seconds, but every atrial impulse still successfully reaches the ventricles, resulting in a regular rhythm.

While the other options describe cardiac rhythm characteristics, they do not specifically define first-degree heart block. Regular QRS complexes indicate that impulses are being conducted through the ventricles appropriately, which is a feature of first-degree block, but it's the extended PR interval that truly qualifies the diagnosis. An irregular heart rhythm or a shortened QT interval are not attributes associated with first-degree heart block, further reinforcing the significance of the PR interval as the defining feature here.